1. Field of the Invention
The invention relates to conduits and pipes generally, and particularly to concrete conduits used to house subterranean electric cable and/or other utility wires.
2. Description of Related Art
Power companies frequently lay electric cables and other utility wires underground. The subterranean placement of power lines offers a variety of well known advantages including public safety. In addition, the need for erecting and placing utility poles is avoided.
One problem with laying power lines in this manner is that the relatively harsh subterranean environment including, for example, silt, water and various pollutants has a tendency to cause corrosion of the plastic insulation surrounding the electric cable. For example, processes have been known to occur which are referred to as electrochemical treeing ("ECT") or water treeing. These processes lead to the development of branched tree-like cracks in the insulation commonly referred to as ECT structures or water trees.
In an attempt to overcome these problems, power companies have required that subterranean cables be housed within concrete conduits to protect the cables from the environment. Power companies have adopted specifications for these concrete conduits which are designed to ensure adequate protection for the cables which they house. For example, the conduit is required to have a square cross section to allow for stacking of multiple conduits on top of one another and a smooth concentric interior bore for receiving cable with minimal friction. The ends of each conduit are provided with extensions for receiving a plastic coupling which functions to join each conduit section in end-to-end fashion with another conduit section. The coupling is typically a polyvinlychloride tube which is force fitted around the relatively abrasive extension ends of adjoining conduits with sufficient pressure so as to prevent foreign matter from entering the conduit. Another important requirement is that the conduits must be free from cracks, broken ends, defects and all irregularities.
A major disadvantage associated with the use of concrete conduits is the very high breakage and cracking rate which is encountered during handling. Some utility companies have estimated that, on the average, 30% or more of all concrete conduits are broken during shipping, handling and storage even before they are used.
In addition, compliance with other power company specifications has proved to be troublesome for conduit manufacturers using conventional pressure molding techniques for cement. For example, the extension ends of the conduit must be physically machined when the concrete is in a semi-solid state after pressure molding.
Other drawbacks to the use of concrete conduits are that they are not completely effective in preventing water vapor from coming into contact with the cables because of the porous nature of the concrete. In an attempt to solve this problem, clay has been incorporated into the concrete to decrease its permeability. However, the use of clay is not desirable because there is a tendency for the clay to break off and contaminate the inside of the conduit.
A further drawback of concrete conduits is that the abrasive interior surface of the bore hinders the pulling of electric cables through the conduit. Because of the difficulty of pulling long sections of power cables through the conduit, power companies have had to interrupt the conduits with frequent pit sections in which the cables are spliced together with separate cable sections, thus increasing electrical resistance and the possibility of electrical failure.
In the manufacture of concrete sewer pipes it has been known to provide a corrosion resistant plastic lining along the inside surface of the pipe to prevent corrosion of the concrete caused by sewage inside the pipe. However, the lining along the inside surface has not reduced the tremendous breakage and cracking rate associated with concrete conduits. In addition, these concrete sewer pipes must be produced by conventional pressure molding techniques followed by a coating process for lining the inside surface with plastic. Coating can be difficult to accomplish, requiring several complicated process steps. For example, see U.S. Pat. Nos. 4,054,473, 4,338,153 and 4,670,315.
Accordingly, it is an object of the present invention to provide a concrete conduit which exhibits virtually no breakage or cracking during shipping and routine handling.
It is another object of the invention to provide a conduit which is virtually impermeable to water vapor.
It is a further object of the invention to provide a method for making a virtually unbreakable conduit which does not require the use of conventional pressure molding.
It is a further object of the invention to provide a conduit for housing electric cable having smooth inner and outer surfaces to facilitate sliding of cables through the conduit and easy handling.